The invention generally relates to hydraulic servo valves and specifically to a pilot-operated servo valve with at least three main-stream ports for mounting into a control block.
Pilot-operated electrohydraulic servo valves of twin- and multi-stage design with more than two main-stream ports are used, e.g. as four-way valves to control the position, speed, and/or force in hydraulic cylinders for linear movement, or position, rotation speed and/or torque in hydraulic motors for rotary movement. In either case the hydraulic device has two displacement chambers, each chamber being coupled to one of the main-stream ports.
These four-way servo valves are conventionally designed as plate-stack valves. A main control valve for the main stage is fitted either directly into a valve housing or into a control sleeve which in turn is inserted into the housing. The openings of the main-stream ports are typically arranged symmetrically relative to the likewise symmetrical main control piston. The main control piston is hydraulically actuated by applying hydraulic pressure to its two end surfaces, one in each of two control chambers defined by end caps flange-mounted onto opposite sides of the valve housing. The control chambers are connected via control bores to a pilot servo valve. Return springs bias the main control piston to a centered position.
There are various known designs for mounting valves in control blocks. For example, there are block mounted servo valves with high flow rates, but these valves have only two main-stream ports and are designed as seat valves. Screw-in block mounted valves with four main-stream ports are also known, but are designed as directional switching control valves and employ direct magnetic actuation.
An issue of particular importance to the practical use of servo valves is that of safety in the event of breakdown or fault in the electrical drive system or in the pilot servo valve. Such faults must not result in an undefined position of the main control piston and thus in uncontrollable movements of the hydraulic device, such as closing movements in presses.
Known multi-stage servo valves of the plate-stack design are constructed with an additional, electrically-actuated directional control or clearance valve disposed between the pilot servo valve and the hydraulic control chambers of the main control piston. In the event of a fault, this directional control valve reverts to a spring-biased, center position, in which the connection to the pilot servo valve is interrupted and the control chambers of the main control piston are fluidically coupled. The main control piston is thus biased by two compression springs to a centered position between two spring plates abutting the housing. To achieve well defined behavior of the cylinder movement when the main control piston is centered, the valve control edges must use positive overlapping (that is, the axial distance between the control edges assigned to a port is greater than the port's axial extent), at least in the direction of the pressure source. As compared to designs using zero-overlapping (that is, the axial distance between the control edges is equal to the port's axial dimension) of the four control edges between pressure source, working ports and tank return circuit, such control edge positive overlap has serious drawbacks as to the positioning accuracy of the cylinder in position-control devices and when the valve is used for pressure regulation.
There is therefore a need for a pilot-operated servo valve that can be effectively block mounted and that has a clearly-defined safety position without sacrificing the good dynamic properties available with zero-overlapping control edges. In particular, there is a need to provide these properties in a servo valve of the construction described above, i.e., with a control piston slidably mounted in a control sleeve that has axially spaced openings for at least three main stream ports, the piston having first and second control edges controlling flow through hydraulic connections between the first and second, and second and third main-stream ports, respectively, and in which the piston's movement is controlled by a pilot valve that selectively supplies pressurized fluid to at least one of two control chambers that act on opposing first and second actuating surfaces of the piston, the pilot valve in turn being in a control loop that takes input from a position transducer coupled to the piston.